Seat Belt Interlock Relay, Late 74-76’ - how does it work? Options

[Superhawk996]

For those of you that despise the seatbelt / starter interlock and the horrific screeching noise the seat belt buzzer makes - let me say I understand completely. As I’ve been playing with this on the bench my wife now hates the noise just as much as I do.

This thread is going to explain how it works and how to bench test it over the coming days.




Credit is owed to @JeffBowlsby , @Bdstone914 , and @Montreal914 for providing me with donor parts that will make this thread possible.

Eventually I will post the schematic of this ridiculous conglomeration of components. I hope to eventually put the circuit in LTSpice to help verify its theoretical operation. I make no warranty as to the accuracy of the schematic - this is just me satisfying my own curiosity.



If you have no interest in my personality defects that would lead me to take on this little project; ignore this thread.

If you have a late car with an in-op system and eventually want to get it working - this thread is for you.

Toward the end I’ll share some ideas on how to get rid of or to change that horrific buzzer noise.

[Superhawk996]

Driver side seat circuits

The driver side seat cushion and belt inputs are:
SF - driver side seat cushion (NO)
GF - driver side seat belt latch switch (NC)

NO = Normally open switch contacts
NC = Normally closed switch contacts

If we look at the schematic we can see that the ground first has to travel through the seat cushion switch. If someone is sitting in the seat, the ground signal then travels over to the seat belt latch / receiver switch. Since the belt receiver switch is a normally closed switch, the ground then travels through the switch to the interlock relay if the seat belt isn’t fastened.

With a ground on input GF - the interlock relay lights the FSB light and the buzzer activates.



If no one is sitting in the vehicle - the ground never makes it to the interlock relay.

Similarly, if the seatbelt is fastened the normally closed receiver switch is opened and the ground cannot make it to the interlock relay and the FSB light and buzzer will be off.

This allows the vehicle to be started from outside the vehicle by leaning over the door. Obviously before days of clutch interlock and such. (IMG:style_emoticons/default/biggrin.gif) People were assumed to be a lot smarter back in those less litigious days

The photo below shows the driver side pins circled in red. The passenger side seat cushion and belt inputs are circled in blue.



The passenger side inputs work exactly the same as the driver side.